In recent years, high-speed and large-capacity systems are being developed for undersea cable systems. Along with the development, a system configuration of the undersea cable systems is also becoming complicated, and cases requiring partial redundancy on the land are also increasing.
In a general undersea cable system, when a route is switched to a redundant route, there has been employed a method of performing switching by transferring information on an automatic switching control signal using a K1 byte and a K2 byte of a synchronous digital hierarchy (SDH) overhead between switching devices with use of a data communication network (DCN) line for transponders. However, in a system configuration in which only a partial section has a redundant configuration, as exemplified by route diversity, a transponder is not installed at an intermediate station, and it is impossible to transfer switching information.
In view of the above, PTL 1 discloses a route switching device, in which, when an abnormality such as communication disruption is detected, a receiving route (A route) in its own device is switched from a currently used route to a redundant route, while a signal level of an optical signal to be output from a currently used route is set low and a signal level of an optical signal to be output from a redundant route is set high in a transmitting route (B route). In a device transmitting and receiving an optical signal to and from a route switching device, a route in each of the A route and the B route is automatically switched from a currently used route to a redundant route because a signal level of an optical signal inputted from the currently used route in the B route is low, and a signal level of an optical signal inputted from the redundant route is high. This makes it unnecessary to install a communication function of a control system for transferring switching information in each route switching device.